Relief image printing plates are used in both flexographic and letterpress processes for printing on a variety of substrates, including paper, corrugated stock, film, foil, and laminates. Relief elements typically include a support layer and one or more layers of photocurable polymer in the form of solid sheets. The printer typically peels a cover sheet from the element to expose the photocurable polymer and places a silver halide photographic negative or some other masking device upon the photopolymer. The printer exposes the negative-bearing element to ultraviolet (UV) light through the negative, thereby causing exposed areas of the element to harden, or cure. After the uncured areas of the element are removed, cured polymer remains as the relief printing surface.
The negatives used in such processes typically are costly items, and the time required for their preparation can be considerable, particularly in those print shops that are not capable of preparing negatives in-house. Moreover, any negative which is used for printing must be nearly perfect. Even minor flaws will be carried through onto each printed item. As a consequence, effort must be expended to ensure that the negative is precisely made. In addition, the negative is usually made with silver halide compounds which are costly and which are also the source of environmental concerns upon disposal.
In the art of flexographic printing, processes have been developed to eliminate the use of the negative, thereby offering significant advantages over previous methods such as, for example, cost efficiency, environmental impact, convenience, and image quality. Many such processes are referred to as direct-to-plate (DTP) processes. One DTP process is disclosed in U.S. Pat. No. 5,846,691 to Cusdin, et al., herein incorporated by reference, which describes formation of a computer-generated negative on a photosensitive printing element by ejecting a negative-forming ink from an ink jet print head directly onto the surface of the printing element. Another such process is disclosed in U.S. Pat. No. 5,925,500 to Yang, et al., herein incorporated by reference, which describes a method of making a laser-imaged printing plate by modifying the slip film with a UV absorber and employing a laser to selectively ablate the slip film. In such methods, the slip film, in effect, becomes the negative as only the areas of the photopolymer to be cured are exposed to actinic radiation. Yet another DTP process is disclosed in U.S. Pat. No. 5,262,275 to Fan, herein incorporated by reference, in which a layer of laser-ablatable infrared radiation sensitive material is disposed upon the surface of the printing element.
DTP technology is significantly different than the conventional plate making technology in a number of respects. DTP plates, for example, typically have a photoablative mask directly on the plate. Also, in DTP technology, face exposure, i.e., a blanket exposure to actinic radiation of the photopolymerizable layer on the side that does (or, ultimately will) bear the relief, is done in air (in the presence of oxygen), whereas, with conventional plates, exposure is typically done in vacuum.
Because face exposure is conducted in the presence of oxygen, there is the potential for excessive exposure of the photocurable layer to oxygen in areas where the masking layer has been removed. This can present problems because the photopolymerization kinetics of many materials in the presence of oxygen are very different from those observed in the absence of oxygen because oxygen is a known free radical scavenger. Hence, oxygen has the effect of inhibiting polymerization of the photocurable material, thus requiring longer exposure times. In addition, oxygen could potentially act as a UV screening agent, resulting in attenuation of the actinic radiation. Generally, this phenomenon is referred to as “oxygen inhibition.”
Oxygen inhibition is typically compounded when so-called “capped” photocurable printing elements are used. Capped photocurable elements have a thin photocurable cap disposed upon the main body of the photocurable material. Typically, with such elements, the relief image formed includes photocurable material from the cap layer. Capped printing elements typically have several significant advantages over uncapped elements in DTP processes. For example, the cap typically has a rough surface that can act as an ink receptive layer, resulting in higher ink densities on the printed substrate. Also, capped plates, because of their longer exposure times, typically enable the user to modify dot shape, resulting in smaller, but more robust dots. The cap layer can also include an image contrast (e.g. green) dye which aids in the inspection of the registered image. The cap itself, however, due to the presence of the dye (and other components), acts as an actinic radiation absorbing layer. Thus, the phenomenon of oxygen inhibition is amplified when imagining capped photocurable printing elements to the extent that relatively long front exposures may be required to hold fine detail dots (i.e., 1% dots on a 150 line).
To decrease front exposure times when processing printing elements with DTP technology such that such times are comparable to those of conventional printing elements, the photo speed (i.e., the speed of photopolymerization) typically is increased to counter the effects of oxygen inhibition. One way to do this is to incorporate oxygen scavengers such as, for example, triphenylphosphine and triphenylphosphite, into the polymer formulation. The addition of oxygen scavengers to the polymer formulation, however, not only decreases the front exposure time, but, also decreases the back exposure time as well.
As used herein, “back exposure” is a blanket exposure to actinic radiation of the photopolymerizable layer on the side opposite that which does (or, ultimately will) bear the relief. This is typically done through a transparent support layer. Such exposure is used to create a shallow layer of polymerized material, herein referred to as a “floor,” on the support side of the photopolymerizable layer. The purpose of the floor is generally to sensitize the photopolymerizable layer and to establish the depth of the relief. Typically, it is desired to have back exposure times greater than 15-30 seconds. In DTP technology, however, increasing the photo speed as described above often results in a back exposure time of less than 30 seconds. Such short back exposure times are undesirable because, for reasons discussed in detail below, variations in the thickness of the floor are typically observed. In turn, a non-uniform floor typically contributes to uneven printing due to variation in the relief across the plate.
Back exposure times can be increased in DTP systems by applying a thin, i.e., 1-2 microns, coating of a UV-absorbing compound between the photopolymerizable layer and the support, or backing, layer. This approach, however, is problematic, as it is difficult to apply the UV-absorbing coating uniformly. This, of course, also creates variations in the thickness of the floor. Also, the coating could interact with the laser and create problems of adhesion.
Accordingly, there is a need in the art for an improved method to produce direct-imaged capped and uncapped flexographic printing plates.